How to Make It
In a large bowl, cover the potatoes with cold water and toss to release the starch. Drain and repeat until the water is clear. Drain the potatoes and pat dry with paper towels. Line a baking sheet with parchment paper. In a deep fryer fitted with a basket or in a heavy large saucepan, heat 3 inches of clarified butter to 250°. Add half the potatoes and fry, stirring occasionally, for 1 minute 30 seconds. Transfer the potatoes to the prepared baking sheet and spread them out. Repeat with the remaining potatoes. Refrigerate the potatoes until cold.
Preheat the oven to 450°. In a medium saucepan, heat the remaining 2 tablespoons of clarified butter. Add the shallots and cook over moderate heat, stirring occasionally, until lightly browned, about 8 minutes. Add the Cognac and cook, stirring occasionally, until nearly reduced by half. Using an immersion blender, puree the sauce until smooth. Add the cream, green peppercorns and 2 1/2 teaspoons of the black peppercorns and simmer over moderately low heat until the cream is slightly thickened, about 5 minutes. Season with fleur de sel and keep warm.
Heat a large cast-iron skillet over high heat. Season the steaks with kosher salt. Reduce the heat to moderate, add the oil to the skillet and swirl to coat. Add the steaks and cook until richly browned, about 2 minutes per side. Transfer the skillet to the oven and roast until an instant-read thermometer inserted in the thickest part registers 120° for rare. Transfer to a carving board to rest.
Meanwhile, line another baking sheet with parchment paper. Heat the clarified butter in the deep fryer to 375°. Add half of the cold potatoes and cook, stirring occasionally, until browned and crisp, 2 to 3 minutes; spread on the prepared baking sheet. Repeat with the remaining potatoes. Season with fleur de sel.
Pour off the fat in the skillet and heat over medium-high heat. Return the steaks to the skillet, add the unsalted butter and cook, tilting the pan and basting the steaks, until the butter is golden brown, about 1 minute. Transfer the steaks to plates, spoon the pepper sauce over the top and season with black pepper. Serve with the French fries. Pass the remaining sauce separately.
Grapeseed oil can replace the clarified butter for deep-frying.